The invention relates to a method of reducing the pressure of high pressure compressed gases without generation of droplets of condensible vapors. It also relates to a device to carry out said process.
Various impurities may be present in a compressed gas stored in a cylinder or the like, such as particles and/or vapors of condensible materials. See for example "Particle analysis in cylinder gas"--H. Y. Wen and G. Kasper--Proceedings--Institute of Environmental Sciences--May 6, 1987.
It is known from the article entitled "A gas filtration system for concentrations of 10.sup.-5 particles/cm.sup.3" from G. KASPER and H. Y. WEN, published in Aerosol Science and Technology 5: 167-185 (1986), how to achieve "totally" particle-free process gases.
Particle analysis is today commonly carried out for a plurality of purposes, usually in conjunction with contamination studies. Since most analyzers operate at ambient pressure, and further since gases, e.g. from cylinders, can be highly compressed (up to about 2 500 psi or more), it is necessary to expand said gases to a low pressure, generally atmospheric pressure, before particle analysis can be carried out.
Up to now, the measurement of the concentration of particles in the gas at low pressure, e.g. atmospheric pressure, has been made by expanding said compressed gas directly from the high pressure of the cylinder to atmospheric pressure (see the first article cited above).
However, as disclosed in the copending application Ser. No. 107,177 filed Oct. 13, 1987, the disclosure of which is, incorporated herein by reference, entitled "Method of detecting trace amounts of condensible vapors from compressed gas"--Kasper et al.--, it has been discovered that under certain circumstances, condensible vapors, even present as trace amounts, may generate droplets of condensed vapors during the expansion of the compressed gas through a critical orifice.
If a pressure regulator which generally comprises at least one critical orifice is used for the expansion of sais compressed gas, it may thus lead to the formation of droplets which will be thereafter detected as particles by the analyzer.
Therefore, it is an object of the present invention to reduce the pressure of highly compressed gases without the introduction of condensation droplets in the expanded gas.
If is another object of the present invention to reduce the presence of highly compressed gas in order to analyze the particles present in said gas, without introducing additional particles.